A report of the Subcommittee on Investigations and Oversight of the Committee on Public Works and Transportation, a committee of the U.S. House of Representatives entitled "The Impact of Weather on Aviation Safety", November 1984, indicates that research relative to the meteorological phenomena identified as a microburst began in 1976 following a June 1975 Eastern Airlines crash at JFK Airport. The microburst is defined as a powerful downward blast of air usually associated with a thunderstorm or rain, followed by a violent horizontal burst of air in all directions, or, wind shear. Microbursts are characterized as relatively small, two and a half miles in diameter or less, shortlived, 95% of microbursts reach maximum intensity in ten minutes and are difficult to detect with existing technology. Notwithstanding the research reportedly begun in 1976, the report, some 8 years later, recognized the need for an improvement in low level wind shear alert systems.
A number of techniques have been advanced for detection of wind shear, see Tanner U.S. Pat. No. 4,043,194 and Fletcher et al U.S. Pat. No. 3,984,685. Radar has been applied to this problem, see Fetter U.S. Pat. No. 4,015,257; Payne U.S. Pat. No. 4,223,309; Lucchi U.S. Pat. No. 4,370,652 and "Measurements Showing the Feasibility for Radar Detection of Hazardous Wind Shear at Airports" by Chadwick et al, a report provided to the National Oceanic and Atmospheric Administration and available under NTIS Accession No. AD A061 596.
Notwithstanding these efforts, insofar as I am informed there is no (presently available or known) reliable method or apparatus for detecting microbursts.
My analysis leads me to conclude that FIG. 1 can be used to describe a typical microburst. FIG. 1 is a cross-section of such a microburst. A region or zone Z of superheated air a distance above the ground is initially penetrated by rain. The rain evaporates at a high rate to quickly reduce the air temperature and thereby increase the density of the superheated air. The now heavy air proceeds to move downwardly at an accelerating rate until it reaches ground level where it must now spread out horizontally. The short-term nature of a microburst is explained since the rapid air fall causes a partial vacuum and thus as the air drops to the ground and is heated, it returns to reduce that partial vacuum. As additional rain penetrates the zone Z, equilibrium results and the microburst terminates. I estimate that the velocity of the falling air can approach dozens of feet per second while the diameter of the falling column may be only a few hundred feet. As a consequence, near ground level we have a velocity profile similar to an inverted mushroom with radial surface winds of up to 50 mph, so that an aircraft flying diametrically through the column would experience a 100 mph air speed change over a distance as short at one mile.
Quantifying some of the geometry of FIG. 1 may provide some insight into the microburst. FIG. 2 shows the area of the falling column (Area #1) is a circular planar area of diameter H. We define the surface of a right cylinder of radius D2, the cylinder having a length or height h2, and refer to this area as Area #2. We define a third area as a right cylinder of diameter D1 and length or height h1. Actual observation indicates that h1 can be 200 feet and we will assume a typical diameter for D1 as 1 mile (5280 feet). On this basis then, what is the area of the falling column, of diameter H? Conservation laws indicate that Area #1=Area #2=Area #3. Thus .pi..times.(H/2).sup.2 =.pi..times.D2.times.h2=.pi..times.D1.times.h1. Rearranging terms, we can write H=2.sqroot.D1.times.h1, or approximately 2000 feet.
The literature cited below indicates some confusion by the poorly defined use of terms such as wind shear, microburst and downburst. That literature includes: Sutton, O. G., Micrometerology, Chapters 1, 7, Krieger, 1977; Offi, D.C., "Wake Vortex Detection with Pulsed-Doppler Radar ASR-8 Feasibility Tests", FAA Technical Center Letter Report, Atlantic City Airport, N.J., February, 1982; Campbell, W. C., Chadwick, R. B., K. B. Earnshaw and K. P. Moran, "Low Elevation Angle Wind Measurements by FW-CW Radar", Proceedings of the 19th Conference in Radar Meteorology, American Meteorological Society, Boston, Mass. April, 1980; Skolnik, Merrill I., Introduction to Radar Systems, Second Edition, McGraw-Hill, 1980; "Wind Shear Microburst Focus of Weather Study", Aviation Week and Space Technology, June 14, 1982; "Wind-Alert System OK'd for 34 Airports", Daily News, Oct. 12, 1979; Covault, Craig, "Heavy Rain Danger Called Greater than Wind Shear", Aviation Week and Space Technology, Jan. 26, 1981; "NTSB Hearing on 727 Crash Expected to Focus on Weather", Aviation Week and Space Technology, Aug. 2, 1982; North, David M., "Weather Conditions Draw Focus in Pan Am Crash", Aviation Week and Space Technology, July 19, 1982; "The Wind-Shear Factor", Newsweek, July 26, 1982; "The Dulles Airport Pressure Jump Detector Array for Gust Front Detection", Bulletin American Meteorological Society, Vol. 58, No. 9, September, 1977; "The Dulles Airport AcousticMicrowave Radar Wind and Wind Shear Measuring System", Bulletin American Meteorological Society, Vol. 58, No. 9, September 1977; "Operational Application of Meteorological Doppler Radar", Bulletin American Meteorological Society, Vol. 61, No. 10, October, 1980; Fujita, T., and F. Caracena, "An Analysis of Three Weather-Related Aircraft Accidents", Bulletin American Meteorological Society, Vol. 58, No. 11, November, 1977; Haltiner, G. J., and F. L. Martin, Dynamical and Physical Meteorology, McGraw-Hill, 1957; Geiger, Rudolf, Climate Near the Ground, Harvard University Press, 1971; Atmospheric Phenomena: Readings from Scientific American, W. H. Freeman, 1980; Nathanson, F. E., Radar Design Principles, McGraw-Hill, 1969 and Anderson, Ray, "Understanding Aeolus", Cruising World, August, 1982.
It is my conclusion that one of the major problems with the microburst is its size, that is, its size may be as important, if not more important, than the severity of wind velocity. This comes about as a natural consequence of the characteristics of large airliners and the training imparted to their pilots. Firstly, during take off and landing maneuvers, the aircraft is typically purposely operated near stall speed (sometimes only 35 mph above stall speed) and the acceleration capabilities of these large craft are inadequate to cope with large air speed variations. As described in the subcommittee report cited above, the first symptom of a microburst is a relatively rapid increase in head wind. This increases the air flow across the wings of the aircraft, increasing its air speed and, if nothing else is changed, would induce the aircraft to rise above the desired flight path. In the absence of accurate knowledge of the wind shear phenomenon, the typical pilot would choose to throttle back (as they are trained to do) so as to reduce air speed to a target air speed. Almost immediately a down draft occurs (i.e. the head wind disappears) and almost immediately thereafter a strong tail wind appears. Because the pilot is now in a powered down mode, increased air speed to counteract the performance decreasing tail wind is required. However, the aircraft does not have the acceleration capabilities to cope with this rapid wind variation and an aircraft stall and subsequent crash can be expected. Based on my analysis, the invention overcomes the prior art problems by looking for a weather phenomenon having the characteristics defined above. Furthermore, it is not necessary to search vast regions of air space, rather the search can be confined to relatively localized areas. More particularly, the severity of the microburst is limited to the regions in which aircraft purposely fly at or near stall speed and at the same time are relatively close to the ground so that there is little time or space within which to recover. For example, I estimate the region of vulnerability to an aircraft to be in those regions where the aircraft is less than approximately 1200 feet high. Using a typical glide slope of about 3.degree. from the horizontal, this region is limited to the aircraft's flight path within 4 miles of its touch down or take off point. This is the preferred region within which a search should be conducted although the search could be limited to an even smaller region, say where the aircraft is less than 300 feet in height which extends along the aircraft flight path about a mile from its touch down or take off point.
Once detected, it is conceivable that a pilot (being warned) could maneuver the aircraft safely through a microburst, particularly by not powering down when first encountering a head wind. Preferably, however, based on the short lifetime of the microburst (10-15 minutes), a microburst which is detected in the region I have defined above will result in terminating aircraft operations through the microburst. This short interruption (10-15 minutes) will provide the maximum safety.
Having defined the region within which the search will be conducted, it is now important to determine how we will detect the presence of a microburst. Relatively conventional doppler radar is capable of identifying the velocity of air in motion to the extent that the velocity is radially directed (parallel to beam propagation). The velocity profile associated with the microburst is in the form of an inverted mushroom, where air is travelling radially outward from the downwardly moving column around the entire 360.degree.. Accordingly, if a microburst is located in the region to be searched, the radar beam will propagate diametrically through the velocity profile, at least at one particular azimuth. At this azimuth the wind velocity will be radial (parallel to beam propagation) and there will be two range cells where velocity is (or nearly is) equal in magnitude and opposite in direction. At nearby azimuths the actual wind velocity will be equal, but the component of wind velocity parallel to the beam propagation path will be smaller. Identification of that beam propagation azimuth which is associated with maximum wind velocity will serve to identify a line along which the center of the microburst lies. One cell or group of range cells along that azimuth will exhibit wind velocity in one sense, a different cell or group of range cells along the same azimuth will indicate wind velocity of equal (or near equal) velocity but opposite sense. The distance between these two groups of range cells then is the diameter of the microburst, and halfway between these two groups of range cells identifies the center of the microburst.
The inventive microburst detector then, employs a doppler radar which may be range and azimuth gated to provide wind velocity information to an intelligent processor from selected regions in space, particularly those regions in which an aircraft would be vulnerable to the microburst. The signals provided to the intelligent processor identify the component of actual wind velocity in the direction of the beam propagation path. The wind velocity components are stored for processing, preferably in an array. The intelligent processor then effects a pattern matching process based on the criteria identified above.
In one particular implementation the data is stored in a array, with each cell storing the sign and magnitude of sensed wind velocity (the component of velocity parallel to the beam propagation path). The array can be considered to have two dimensions, different ranges are identified as different rows in the array, and the other dimension corresponds to the columns of the array. There is a unique correspondence between a particular cell and a real region within the monitored volume. The microburst can be detected and located by a pattern matching operation. One particular row (range) is selected and the data stored therein are compared to the data stored in different rows (comparing cells in like columns). When a pair of rows are identified in which the stored wind velocity in at least one pair of corresponding cells in different rows is equal and opposite (to within some difference threshold), a first step in the pattern matching has been accomplished because the center of the microburst may lie in the row midway between the two selected rows and equi-distant therefrom. Since the doppler radar senses only velocity components parallel to the propagation path, we realize that a pair of cells lying on the diameter of the circle defining the microburst location will exhibit equal (and maximum) and opposite velocity. Cells on the circumference of that circle and on the perpendicular diameter will show zero velocity since, at those cells, wind velocity is perpendicular to beam propagation. This is another criteria that can be used in the pattern matching process. This criteria is not preferred for it relies on comparisons of near zero data and thus could not be expected to be particularly accurate.
An alternative to locate the center of the microburst we compare sense and magnitude of wind velocity between different cells in a common row, but preferably not the row suspected to contain the center of the microburst. Identifying pairs of cells in a common row which exhibit equal magnitude and sense of wind velocity allows us to pinpoint the center of the microburst as lying in the column midway between the selected cells which exhibit this characteristic.
Accordingly, the invention, in one embodiment, provides a microburst detector including a doppler radar transmitting along a propagation path varying in range and azimuth across a region in space including a termination of an airport runway and, an intelligent processor responsive to signals from the doppler radar,
the intelligent processor including storage means for storing data representative of wind velocity components parallel to said propagation path in a predetermined order, said intelligent processor further including:
pattern matching means for searching for a specific pattern of said data for locating a center of a particular circle wherein data of equal and opposite sign are stored in a pair of cells located along one diameter of the particular circle, and
means for signalling a microburst on detection of said pattern.
Once a microburst has been detected as aforesaid, its location and radial extent can be readily determined. The center is merely a point defined by the intersection of the diameter referred to above and another diameter which intersects the circle at locations where null or near null wind velocity data is stored. If we examine the data stored in the cells located radially outward of the center and on the diameter parallel to the beam path, we will find a pattern of continually increasing velocity to a maximum and then a decrease in the velocity. The distance between the center and the velocity maximum determine the radius of the microburst. By monitoring the same region as a function of time, we can detect movement of the microburst as well as its termination phase.
While the life of a microburst is, on the meteorological scale, relatively minor (10-15 minutes), on the scale of operations taking place in a suitable embodiment of an intelligent processor, the characteristics of the microburst and its movement do not occur very rapidly. My present estimate is that it will be entirely sufficient to accumulate data in a single radar sweep (for example 1 second), arrange the intelligent processor so that it need not process data in less than 60 seconds. Accordingly, new data is present only once per minute.